This invention relates to a dynamoelectric machine having a quiet cooling system. A typical example of the state of the art in cooling systems for dynamoelectric machines prior to this invention is the cooling system for a totally enclosed fan cooled motor. That system comprises a motor having cooling fins axially disposed about its periphery, a rotor shaft extending beyond the body of the motor, blades disposed on a hub which is clamped to the extended portion of the rotor shaft, a casing which totally encloses the motor, and a protective grille attached to the motor casing. In these motors the air flow enters axially across a high resistance grille, abruptly turns ninety degrees, is forced radially into a large void region, again turns ninety degrees back to the axial direction, and exits the flow system through jets to the cooling fins. The high resistance grille and the two ninety degree turns cause turbulence which causes pressure loss which reduces the coolant flow rate resulting in lower cooling effectiveness. Similarly, the large void regions at the exit of the blower permit recirculation, turbulence, and flow separation which likewise reduce the cooling effectiveness of the system. In addition, because a direct relation exists between the turbulence, flow separation, and sound pressure levels; turbulence or flow separation result in a noisier machine.
Also characteristic of the prior art fans is their low number of blades. In the prior art, the turbulence and flow separation not only cause high noise levels, but the low blade number produces a harsher noise which is audibly noticeable and contributes to greater subjective annoyance.
In U.S. Pat. No. 3,025,799, to S. L. Hallerback, issued Mar. 20, 1962, which does not deal specifically with noise treatment, there is described an immersible drainage pump. In the Hallerback patent the flow enters radially (with respect to the rotor axis) through a strainer and proceeds into a large void region where turbulence and recirculation occur. The flow must then turn ninety degrees into the blades of the pump. From there the flow is conducted through a substantially radial contour from which the flow must make a sharp turn to exit in an axial direction. The concave curvature of the hub provides for a substantially radial flow. The large void regions, the sharp turns, and the blunt hub nose all contribute to turbulence which causes noise. The Hallerback patent describes neither the number nor type of blades used.
In U.S. Pat. No. 2,886,721 to V. J. Picozzi et al, issued May 12, 1959, there is described an acoustically treated motor for reducing the overall noise level and quality of noise emanating from the machine during operation. The Picozzi invention primarily involves mounting a dissipative type muffler on a motor consisting of a plurality of sound insulated plenum chambers being located in a path followed by circulating air used in ventillating the machine. The tortuous ventillating path of the Picozzi patent is specifically designed so that there is no direct path for noises generated in the machine to escape to the atmosphere. As Picozzi states, fans produce two distinct kinds of aerodynamic noises, one of which, broad band noise, contains all frequencies arbitrarily distributed throughout the noise spectrum. The other kind is the discrete or pure tone noise which is individual noise of a single frequency. The broad band noise being miscellaneous and random, is not as annoying to the listener as the pure tone noise. On the other hand, pure tone noise creates greater subjective annoyance because most of the sound energy is concentrated into a single tone at a relatively high frequency. Picozzi proposed increasing the blade number from four to as many as twenty-one or twenty-three. His objective was to increase the pure tone frequency the amplitude of which is then easier to muffle. Increasing the frequency actually increases the subjective annoyance; however, Picozzi proposed to muffle these noises. The Picozzi patent does not teach a method of eliminating pure tone noises other than by muffling them.
U.S. Pat. No. 2,881,337, to R. L. Wall, issued Apr. 7, 1959, is primarily concerned with acoustical treatment of motors by providing a sound absorbent device of the resonant type associated with the motor for attenuating frequencies of high decibel levels.
An example of a fan cooling system is the 364 frame size 60 horsepower totally enclosed fan cooled 4-pole motor manufactured by ASEA of Sweden. Although this fan system is conical in some respects, the open front face with internal blading does not provide the proper inlet air approach to the blade row which results in an increased noise level. Likewise, the relative position of the fan hub to the motor frame causes disruption of the flow of air from the hub rather than providing a controlled guidance into the cooling fin channels thereby increasing the noise level. Due to the low blade number (10), the ASEA fan noise spectrum is characterized by significant pure tone noise which is undesirable. In addition, the conical portion of the shroud does not provide a decreasing annulus height relative to the shroud, so that a proper rate of diffusion is not maintained.